A 60-year-old patient with a history of ischaemic heart disease and heart failure (LVEF 35%) is being treated with ramipril, bisoprolol, and furosemide. His symptoms of breathlessness on exertion persist. His potassium is 4.2 mmol/L and eGFR is 55 mL/min/1.73m². According to NICE CG108 (Chronic heart failure), which medication should be considered next to improve prognosis and symptoms?